THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF NATURE.
The first impression which is produced upon the mind, when the
phenomena of volcanic action are studied, is that here we have
exhibitions of destructive violence the effects of which must be
entirely mischievous and disastrous to the living beings occupying the
earth's surface. A little consideration will convince us, however, that
the grand and terrible character of the displays of volcanic energy
have given rise to exaggerated notions concerning their destructive
effects. The fact that districts situated over the most powerful
volcanic foci, like Java and Japan, are luxuriant in their productions,
and thickly inhabited, may well lead us to pause ere we condemn
volcanic action as productive only of mischief to the living beings
on the earth's surface. The actual slopes of Vesuvius and Etna, and
many other active volcanoes, are abundantly clothed with vineyards and
forests and are thickly studded with populous villages.
As a matter of fact, the actual amount of damage to life and property
which is effected by volcanic eruptions is small. Usually, the
inhabitants of the district have sufficient warning to enable them
to escape with their lives and to carry away their most valuable
possessions. And though fertile tracts are covered by loose dust and
ashes, or by lava- and mud-currents, yet the sterility thus produced
is generally of short duration, for by their decomposition volcanic
materials give rise to the formation of the richest and most productive
soils.
Earthquakes, as we have already seen, are far more destructive in
their effects than are volcanoes. Houses and villages, nay even entire
cities, are, by vibrations of portions of the earth's crust, reduced to
heaps of ruins, and famines and pestilences too frequently follow, as
the consequence of the disorganisation of our social systems by these
terrible catastrophes.
It may well be doubted, however, whether the annual average of
destruction to life and property caused by all kinds of subterranean
action, exceeds that produced either by floods or by hurricanes. Yet
we know that the circulation of water and air over our globe are
beneficial and necessary operations, and that the mischief occasionally
wrought by the moving bodies of water and air is quite insignificant
compared with the good which they effect.
In the same way, we shall be able to show that the subterranean
energies are necessary to the continued existence of our globe as
a place fitted for the habitation of living beings, and that the
mischievous and destructive effects of these energies bear but a small
and insignificant proportion to the beneficial results with which they
must be credited.
[Sidenote: LEVELLING ACTION OF DENUDING FORCES.]
We have had frequent occasion in the preceding pages to refer to the
work--slow but sure, silent but effective--wrought by the action of
the denuding forces ever operating upon the surface of our globe. The
waters condensing from the atmosphere and falling upon the land in
the form of rain, snow, or hail, are charged with small quantities of
dissolved gases, and these waters penetrating among the rock-masses
of which the earth's crust is composed, give rise to various
chemical actions of which we have already noticed such remarkable
illustrations in studying the ancient volcanic products of our globe.
By this action the hardest and most solid rock-masses are reduced
to a state of complete disintegration, certain of their ingredients
undergoing decomposition, and the cementing materials which hold
their particles together being removed in a state of solution. In the
higher regions of the atmosphere this work of rock-disintegration
proceeds with the greatest rapidity; for there the chemical action is
reinforced by the powerful mechanical action of freezing water. On
high mountain-peaks the work of breaking up rock-masses goes on at the
most rapid rate, and every craggy pinnacle is swathed by the heaps
of fragments which have fallen from it. The Alpine traveller justly
dreads the continual fusillade of falling rock-fragments which is
kept up by the ever-active power of the frost in these higher regions
of the atmosphere; and fears lest the vibrations of his footsteps
should loosen, from their position of precarious rest, the rapidly
accumulating piles of detritus. No mountain-peak attains to any very
great elevation above the earth's surface, for the higher we rise in
the atmosphere the greater is the range of temperature and the more
destructive are the effects of the atmospheric water. The moon, which
is a much smaller planet than our earth, has mountains of far greater
elevation; but the moon possesses neither an atmosphere nor moisture on
its surface, to produce those levelling effects which we see everywhere
going on around us upon the earth.
The disintegrated materials, produced by chemical and mechanical
actions of the atmospheric waters upon rock-masses, are by floods,
rivers, and glaciers, gradually transported from higher to lower
levels; and sooner or later every fragment, when it has once been
separated from a mountain-top, must reach the ocean, where these
materials are accumulated and arranged to form new rocks.
Over every part of the earth's surface these three grand operations of
the disintegration of old rock-masses, the transport of the materials
so produced to lower levels, and the accumulation of these materials
to form new rocks, is continually going on. It is by the varied action
of these denuding agents upon rocks of unequal hardness, occupying
different positions in relation to one another, that all the external
features of hills, and plains, and mountains owe their origin.
It is a fact, which is capable of mathematical demonstration, that by
the action of these denuding forces the surface of all the lands of the
globe is being gradually but surely lowered; and this takes place at
such a rate that in a few millions of years the whole of the existing
continents must be washed away and their materials distributed over the
beds of the oceans.
[Sidenote: NECESSITY FOR COMPENSATING AGENCIES.]
It is evident that there exists some agency by which this levelling
action of the denuding forces of the globe is compensated; and a little
consideration will show that such compensating agency is found in the
subterranean forces ever at work within the earth's crust. The effects
of these subterranean forces which most powerfully arrest our attention
are volcanic outbursts and earthquake shocks, but a careful study of
the subject proves that these are by no means the most important of the
results of the action of such forces. Exact observation has proved that
almost every part of the earth's surface is either rising or falling,
and the striking and destructive phenomena of volcanoes and earthquakes
probably bear only the same relation to those grand and useful
actions of the subterranean forces, which floods do to the system of
circulating waters, and hurricanes to the system of moving air-currents.
If we ride in a well-appointed carriage with good springs, upon
a railway which is in excellent order, the movement is almost
imperceptible to us; and the rate of speed may be increased
indefinitely, without making itself apparent to our senses. The
smallest impediment to the evenness of the movement--such as that
produced by a small object placed upon the rails--at once makes itself
felt by a violent jar and vibration. How perfectly insensible we may
be of the grandest and most rapid movements is taught us by the facts
demonstrated by the astronomer. By the earth's daily rotation, we are
borne along at a rate which in some places amounts to over 1,000 miles
an hour; and by its annual revolution we are every hour transported
through a distance of 70,000 miles; yet concerning the fact and
direction of these movements we are wholly unconscious.
In the case both of the railway train and of our planet, we can only
establish the reality of the movement, and its direction and rate, by
means of observations upon external objects, which appear to us to
have a movement in the opposite direction. In the same way we can only
establish the fact of the movement of portions of the earth's crust
by noticing the changing positions of parts of the earth's surface in
relation to the constant level of the ocean. When this is done we find
abundant proof that while some parts of the earth's crust are rising,
others are as undoubtedly undergoing depression.
[Sidenote: POTENCY OF THE SUBTERRANEAN FORCES.]
We shall be able to form some idea of the vastness of the effects
produced by the subterranean forces, by a very simple consideration.
It is certain that during the enormous periods of time of which the
records have been discovered by the geologist, there have always been
continents and oceans upon the earth's surface, just as at present,
and it is almost equally certain that the proportions of the earth's
surface occupied by land and water respectively, have not varied very
widely from those which now prevail. But, at the same time, it is an
equally well-established bet that the denuding forces ever at work
upon the earth's surface would have been competent to the removal
of existing continents many times over, in the vast periods covered
by geological records. Hence we are driven to conclude that the
subterranean movements have in past times entirely compensated for the
waste produced by the denuding forces ever at work upon our globe. But
this is not all. The subterranean forces not only produce upheaval;
in a great many cases the evidences of subsidence are as clear and
conclusive as are those of upheaval in others. Hence we are driven to
conclude that the forces producing upheaval of portions of the earth's
crust are sufficient, not only to balance those producing subsidence,
but also to compensate for the destructive action of denuding agents
upon the land-masses of the globe.
It is only by a careful and attentive study and calculation of the
effects produced by the denuding agents at work all around us, aided
by an examination of the enormous thicknesses of strata formed by the
action of such causes during past geological times, that we are able
to form any idea of the reality and vastness of the agents of change
which are ever operating to modify the earth's external features. When
we have clearly realised the grand effects produced on the surface
of the globe by these external forces, through the action of its
investing atmosphere and circulating waters, then, and only then,
shall we be in a position to estimate the far greater effects resulting
from the internal forces, of which the most striking, but not the most
important, results are seen in the production of volcanic eruptions and
earthquake-shocks.
Another series of facts which serve to convince the geologist of the
reality and potency of the forces ever at work within the earth's
crust, and the way in which these have operated during past geological
periods, is found in the disturbed condition of many of the stratified
rock-masses of which it is composed. Such stratified rock-masses, it is
clear, must have been originally deposited in a position of approximate
horizontality; but they are now often found in inclined and even
vertical positions; they are seen to be bent, crumpled, puckered, and
folded in the most remarkable manner, and have not unfrequently been
broken across by dislocations--'faults'--which have sometimes displaced
masses, originally in contact, to the extent of thousands of feet.
The slate-rocks of the globe, moreover, bear witness to the fact that
strata have been subjected to the action of lateral compression of
enormous violence and vast duration; while in the metamorphic rocks
we see the effects of still more extreme mechanical strains, which
have been in part transformed into chemical action. No one who has
not studied the crushed, crumpled, fractured, and altered condition
of many of the sedimentary rocks of the globe, can form the faintest
idea of the enormous effects of the internal forces which have been in
operation within the earth's crust during earlier geological periods.
And it is only by such studies as these that we at last learn to regard
the earthquake and volcanic phenomena of our globe, not as the grandest
and most important effects of these forces, but as their secondary
and accidental accompaniments. 'Volcanoes,' it has been said, 'are
the safety-valves of the globe;' and when we come to realise the real
extent and nature of the internal forces ceaselessly working in the
earth's crust we shall scarcely be disposed to regard the simile as an
overstrained one.
[Sidenote: RELATION TO CONTINENTAL MOVEMENTS.]
The first geologist who attempted to show the exact relations existing
between those subterranean forces which cause the movements of
continental masses of land, and those more startling displays of energy
which are witnessed in volcanic outbursts, was the late Mr. Poulett
Scrope. At a somewhat later date Mr. Darwin, in his remarkable paper
'On the Connexion of certain Volcanic Phenomena in South America, and
on the Formation of Mountain-chains and Volcanoes as the effect of
Continental Elevations,' threw much new and important light upon the
question.
While, on the one hand, we are led by recent geological investigations
to reject the notions which were formerly accepted, by which
mountain-ranges were supposed to be suddenly and violently upheaved by
volcanic forces, we are, on the other hand, driven to conclude that
without the action of these subterranean forces, the irregularities
which are exhibited on the earth's surface could not have had any
existence.
It is true that the actual forms of the mountain-ranges are due
directly to the action of denuding forces, which have sculptured
out from the rude rocky masses all the varied outlines of peaks and
crags, of ravines and valleys. But it is none the less true that
the determining causes which have directed and controlled all this
earth-sculpture, are found in the relative positions of hard and soft
masses of rock; but these rock-masses have acquired their hardness and
consistency, and have assumed their present positions, in obedience
to the action of subterranean forces. Hence we see that though the
formation of mountain-ranges is proximately due to the denuding
forces, which have sculptured the earth's surface, the primary cause
for the existence of such mountain-chains must be sought for in the
fact that subterranean forces have been at work, folding, crumpling,
and hardening the soft sediments, and placing them in such positions
that, by the action of denudation, the more indurated portions are left
standing as mountain-masses above the general surface.
The old notion that mountain-chains are due to a vertical upthrust
from below, finds but little support when we come to study with due
care the positions of the rock-masses composing the earth's crust. On
the contrary, we find that mountain-ranges are usually carved out of
the crushed and crumpled edges of strata which have along certain lines
been influenced by great mechanical strains, and subjected to more or
less induration and chemical alteration. When we compare these folded
and contorted portions of the strata with those parts of the same beds
which are not so affected, we find the effects produced in the former
are not such as would result from an upthrust from below, but from
movements by which a tangential strain would be brought about. If we
imagine certain lines of weakness to exist in the solid crust of the
earth, then any movements in the portions of the crust between these
lines of weakness would cause crushing and crumpling of the strata
along the latter.
[Sidenote: FORMATION OF MOUNTAIN-CHAINS.]
Recent investigations of Dana and other authors have thrown much new
light upon the question of the mode of formation of mountain-chains,
and the relation between the movements by which they are produced and
the sudden and violent manifestations of force witnessed in volcanic
outbursts. We cannot, perhaps, better illustrate this subject than by
giving a sketch of the series of operations to which the great Alpine
chains owe their origin.
There are good grounds for believing that the great mountain-axis of
Southern Europe, with its continuation in Asia, had no existence
during the earlier geological periods. Indeed, it has been proved that
all the higher among the existing mountain-chains of the globe have
been almost entirely formed in Tertiary times. The reason of this
remarkable fact is not far to seek. So rapid is the work of denudation
in the higher regions of the atmosphere, that the elevated crags and
pinnacles are being broken up by the action of moisture and frost at
an exceedingly rapid rate. This fact is attested by the existence
of those enormous masses of angular rock-fragments which are found
lodged on every vantage-ground among the mountain-summits, as well as
by the continually descending materials which are borne by glaciers
and mountain-torrents to the valleys below. Where such a rate of
disintegration as this is maintained, no elevated mountain-crests
could exist through long geological periods. It is true we find in all
parts of the globe relics of many mountain-chains which were formed
before the Tertiary period; but these have by long-continued denudation
been worn down to 'mere stumps.' Of such worn-down and degraded
mountain-ranges we have examples in the Scandinavian chains, and some
of the low mountain-regions of Central Europe and North America.
Let us now proceed to illustrate this subject by briefly sketching the
history of that series of operations by which the great mountain-chains
of the Alpine system have been formed.
The first stage of that grand series of operations appears from recent
geological researches to have consisted in the opening of a number
of fissures running along a line near to that at which, in a long
subsequent period, the elevation of the mountain-masses took place.
This betrayal of the existence of a line of weakness in this part of
the earth's crust occurred in the Permian period, and from that time
onward a series of wonderful movements and changes have been going
forward, which have resulted in the production of the Alpine chains as
we now see them.
[Sidenote: VOLCANIC FISSURES OF PERMIAN PERIOD.]
From the great fissures opened in Permian times along this line of
weakness, great quantities of lava, scoriæ, and tuff were poured out,
and these accumulated to form great volcanic mountains, which we can
now only study at a few isolated spots, as in the Tyrol, Carinthia,
and about Lake Lugano. Everywhere else, these Permian volcanic rocks
appear to be deeply buried under the later-formed sediments, from which
the Alpine chains have been carved. Few and imperfect, however, as are
the exposures of these ancient rhyolite and quartz-andesite lavas and
agglomerates formed at the close of the Palæozoic epoch, their greatly
denuded relics form masses which are in places more than 9,000 feet in
thickness. From this fact we are able to form some slight idea of the
scale upon which the volcanic outbursts in question must have taken
place during Permian times.
The second stage in the series of operations by which the Alpine chains
have been formed, consisted in a general sinking of the surface along
that line of weakness in the earth's crust, the existence of which had
been betrayed by the formation of fissures and the eruption of volcanic
rocks. We have already had occasion to remark how frequently such
subsidences follow upon the extrusion of volcanic masses at any part
of the earth's surface; and we have referred these downward movements
in part to the removal of support from below the portion of the crust
affected, and in part to the weight of the materials piled upon its
surface by the volcanic forces.
The volcanic energy which had been manifested with such violence
during the Permian period, does not appear to have died out altogether
during the succeeding Triassic period. A number of smaller volcanic
vents were opened from time to time, and from these, lavas, tuffs, and
agglomerates, chiefly of basic composition, were poured out. The relics
of these old Triassic volcanoes are found at many points along the
Alpine chain, but it is evident that the igneous forces were gradually
becoming exhausted during this period, and before the close of it they
had fallen into a state of complete extinction.
But the great subsidence which had commenced in the Triassic period,
along what was to become the future line of the Alpine chain, was
continued almost without interruption during the Rhætic, the Jurassic,
the Tithonian, the Neocomian, the Cretaceous and the Nummulitic
periods. With respect to the strata formed during all these periods,
it is found that their thiknesses, which away from the Alpine axis
may be measured by hundreds of feet, is along that axis increased
to thousands of feet. The united thickness of sediments accumulated
along this great line of subsidence between the Permian and Nummulitic
periods probably exceeds 60,000 feet, or ten miles. The subsidence
appears to have been very slow and gradual, but almost uninterrupted,
and the deposition of sediments seems to have kept pace with the
sinking of the sea-bottom, a fact which is proved by the circumstance
that nearly the whole of these sediments were such as must have been
accumulated in comparatively shallow water.
[Sidenote: FORMATION OF ALPINE GEOSYNCLINAL.]
By the means we have described there was thus formed a 'geosynclinal,'
as geologists have called it, that is, a trough-like hollow filled
with masses of abnormally thickened sediments, which had been piled
one upon another during the long periods of time in which almost
uninterrupted subsidence was going on along the Alpine line of
weakness in the earth's crust. In this way was brought together that
enormous accumulation of materials from which the hard masses of the
Alpine chains were subsequently elaborated, and out of which the
mountain-peaks were eventually carved by denudation.
The third stage in this grand work of mountain-making commenced in the
Oligocene period. It consisted of a series of movements affecting the
parts of the earth's crust on either side of the line of weakness
which had first exhibited itself in Permian times. By these movements
a series of tangential strains were produced, which resulted in the
violent crushing, folding, and crumpling of the sedimentary materials
composing the geosynclinal.
One effect of this action was the violent flexure and frequent fracture
of these stratified masses, which are now found in the Alpine regions
assuming the most abnormal and unexpected positions and relations to
one another. Sometimes the strata are found tortured and twisted into
the most complicated folds and puckerings; at others they are seen to
be completely inverted, so that the older beds are found lying upon the
newer; and in others, again, great masses of strata have been traversed
by numerous fractures or faults, the rocks on either side of which are
displaced to the extent of thousands of feet.
Another effect of the great lateral thrusts by which the thick
sedimentary masses of the geosynclinal were being so violently
disturbed, was the production of a great amount of induration and
chemical change in these rocks. Masses of soft clay, of the age of
that upon which London is built, were by violent pressure reduced to
the condition of roofing-slate, similar to that of North Wales. One
of the most important discoveries of modern times is that which has
resulted in the recognition of the fact of the mutual convertibility
of different kinds of energy. We now know that mechanical force
may be transformed into heat-force or chemical force; and of such
transformations we find abundant illustrations in the crushed and
crumpled rock-masses of the Alpine chains.
Under the influence of these several kinds of force, not only was
extreme consolidation and induration produced among the rock-masses,
but chemical affinity and crystalline action had the fullest play
among the materials of which they were composed. In many cases we find
the originally soft muds, sands, and shell-banks converted into the
most highly crystalline rocks, which retain their primary chemical
composition, but have entirely lost all their other original features.
[Sidenote: FORMATION OF ALPINE GEANTICLINAL.]
To the mass of folded, crumpled, and altered strata, formed from a
geosynclinal by lateral pressure, geologists have given the name
of a 'geanticlinal.' The formation of the Alpine geanticlinal was
due to movements which commenced in the Oligocene period, attained
their maximum in the Miocene, and appear to have declined and almost
altogether died out in the Pliocene period.
The movements which resulted in the crushing and crumpling of the
thickened mass of sediments along the Alpine line of weakness, also
gave rise to the formation of a series of fissures from which volcanic
action took place. These fissures were not, however, formed along the
original line of weakness, for this had been strengthened and repaired
by the deposition of ten-miles' thickness of sediments upon it, but
along new fissures opened in directions parallel to the original lines
of weakness, and in areas where a much less considerable amount of
deposition had taken place since Permian times.
We have abundant evidence that, just at the period when those great
movements were commencing which resulted in the formation of the great
Alpine and Himalayan geanticlinal, earth-fissures were being opened
upon either side of the latter from which volcanic outbursts took
place. At the period when the most violent mountain-forming movements
occurred, these fissures were in their most active condition, and at
this time two great volcanic belts stretched east and west, on either
side of, and parallel to, the great Alpine chain. The Northern volcanic
band was formed by the numerous vents, now all extinct, in Auvergne,
Central Germany, Bohemia, and Hungary, and was probably continued in
the volcanoes of the Thian Shan and Mantchouria. The Southern volcanic
band was formed by the numerous vents of the Iberian and Italian
peninsulas, and the islands of the Mediterranean, and were continued
to the eastward by those of Asia Minor, Arabia, and the North Indian
Ocean. As the earth-movements which produced the geanticlinal died
away, the volcanic energy along these parallel volcanic bands died
away at the same time. In studying the geology of Central and Southern
Europe, no fact comes out more strikingly than that of the synchronism
between the earth-movements by which the geanticlinal of the Alps was
formed, and the volcanic manifestations which were exhibited along
lines of fissure parallel to that geanticlinal. The earth-movements
and the volcanic outbursts both commenced in the Oligocene period,
gradually attained their maximum in the Miocene, and as slowly declined
in the Pliocene.
[Sidenote: SCULPTURING OF ALPS BY DENUDATION.]
The fourth stage in the great work of mountain-building in the case
of the Alps consisted in the operation of the denuding forces, the
disintegrating action of rain and frost, the transporting action
of rivers and glaciers, by which the Alpine peaks were gradually
sculptured out of the indurated and altered masses constituting the
geanticlinal. The action of this fourth stage went on to a great
extent side by side with that of the third stage. So soon as the
earth-movements had brought the submerged sedimentary masses of the
geosynclinal under the action of the surface tides and currents of the
ocean, marine denudation would commence; and, as the work of elevation
went on, the rock-masses would gradually be brought within the reach
of those more silently-working but far more effective agents which
are ever operating in the higher regions of the atmosphere. It is
impossible to say what would have been the height of the Alpine chain
if the work of denudation had not to a great extent kept pace with
that of elevation. Only the harder and more crystalline masses have
for the most part escaped destruction, and stand up in high craggy
summits; while flanking hills, like the well-known Rigi, are Been to
be composed of conglomerates thousands of feet in thickness, composed
of their disintegrated materials. It is a remarkable fact, as showing
how enormous was the work of elevation daring the formation of the
geanticlinal, that some of the youngest and least consolidated rocks of
the Nummulitic period are still found at a height of 11,000 feet in the
Alps, and of 16,000 feet in the Himalaya.
From what has been said, it will be seen that mountain-chains may be
regarded as cicatrised wounds in the earth's solid crust. A line of
weakness first betrays itself at a certain part of the earth's surface
by fissures, from which volcanic outbursts take place; and thus the
position of the future mountain-chain is determined. Next, subsidence
during many millions of years permits of the accumulation of the raw
materials out of which the mountain-range is to be formed; subsequent
earth-movements cause these raw materials to be elaborated into the
hardest and most crystalline rock-masses, and place them in elevated
and favourable positions; and lastly, denudation sculptures from these
hardened rock-masses all the varied mountain forms. Thus the work of
mountain-making is not, as was formerly supposed by geologists, the
result of a simple upheaving force, but is the outcome of a long and
complicated series of operations.
[Sidenote: ORIGIN OF OTHER MOUNTAIN-CHAINS.]
The careful study of other mountain-chains, especially those of the
American continent, has shown that the series of actions which we
have described as occurring in the Alps, took place in the same order
in the formation of all mountain-masses. It is doubtful whether the
line of weakness is always betrayed in the first instance by the
formation along its course of volcanic fissures. But in all cases we
have evidence of the production of a geosynclinal, which is afterwards,
by lateral pressure, converted into a geanticlinal, and from this the
mountain-chains have been carved by denudation. Professor Dana has
shown that the geosynclinal of the Appalachian chain was made up of
sediments attaining a thickness of 40,000 feet, or eight miles; while
Mr. Clarence King has shown that a part of the geosynclinal of the
Rocky Mountains was built up of no less than 60,000 feet, or twelve
miles of strata.
It has thus been established that a very remarkable relation exists
between the forces by which continental masses of land are raised
and depressed, and mountain-ranges have been developed along lines
of weakness separating such moving continental masses, and those
more sudden and striking manifestations of energy which give rise to
volcanic phenomena. It is in this relation between the widespread
subterranean energies and the local development of the same forces
at volcanic vents, that we must in all probability seek for the
explanation of those interesting peculiarities of the distribution
of volcanoes upon the face of the globe which we have described
in a former chapter. The parallelism of volcanic bands to great
mountain-chains is thus easily accounted for; and in the same way
we may probably explain the position of most volcanoes with regard
to coast-lines. We have already pointed out the objections to the
commonly-received view that volcanoes depend for their supplies of
water on the proximity of the ocean. This proximity of the ocean to
volcanic vents we are thus inclined to regard, not as the cause, but as
the effect of the subterranean action. The positions of both volcanoes
and coast-lines are determined by the limits of those great areas of
the earth's crust which are subjected to slow vertical movements, often
in opposite directions.
Terrible and striking, then, as are the phenomena connected with
volcanic action, such sudden and violent manifestations of the
subterranean energy must not be regarded as the only, or indeed the
chief, effects which they produce. The internal forces continually
at work within the earth's crust perform a series of most important
functions in connection with the economy of the globe, and were the
action of these forces to die out, our planet would soon cease to be
fit for the habitation of living beings.
There is no fact which the geological student is more constantly
called upon to bear in mind than that of the potency of seemingly
insignificant causes which continue in constant operation through long
periods of time. Indeed these small and almost unnoticed agencies at
work upon the earth's crust are often found, in the long ran, to
produce far grander effects than those of which the action is much
more striking and obvious. It is to the silent and imperceptible
action of atmospheric moisture and frost that the disintegration of
the solid rock-masses must be mainly ascribed; and the noisy cataract
and ocean-billow produce effects which are quite insignificant
compared with those which must be ascribed to the slight and almost
unnoticed forces. Great masses of limestone are built up of the remains
of microscopic organisms, while the larger and higher life-forms
contribute but little to the great work of rock-building.
[Sidenote: EFFECTS OF SLOW CONTINENTAL MOVEMENTS.]
In the same way it is to the almost unnoticed action of the
subterranean forces in raising some vast areas of the earth's crust,
in depressing others, and in bringing about the development of
mountain-chains between them, that we must ascribe a far more important
part in the economy of our globe than to the more conspicuous but less
constant action of volcanoes.
A few simple considerations will serve to convince us, not only of the
beneficial effects of the action of the subterranean energies within
the earth's crust, but of the absolute necessity of the continued
operation of those energies to the perpetuation of that set of
conditions by which our planet is fitted to be the habitation of living
beings.
We have already referred to the prodigious effects which are constantly
being produced around us by the action of the external forces at work
upon the globe. The source of these external forces is found in the
movements and changes which are ever going on within the aqueous and
atmospheric media in which the globe is enveloped. The circulation of
the air, influencing the circulation of the waters in the shape of
clouds, rain, snow, rivers, glaciers, and oceans, causes the breaking
up of even the hardest rock-masses, and the continual removal of their
disintegrated fragments from higher to lower levels. This work goes on
with more or less regularity over every part of the land raised above
the level of the ocean, but the rate of destruction in the higher
regions of the atmosphere is far more rapid than at lower levels. Hence
the circulating air and water of the globe are found to be continually
acting as levellers of the land-masses of the earth.
It is by no means a difficult task to calculate the approximate rate
at which the various continents and islands are being levelled down,
and such calculations prove that in a very few millions of years the
existing forces operating upon the earth's surface would reduce the
whole of the land-masses to the level of the ocean.
But a little consideration will convince us that the circulation of
the air and waters of the globe are themselves dependent upon the
existence of those irregularities of the land-surfaces which they are
constantly tending to destroy. Without elevated mountain ridges the
regular condensation of moisture, and its collection and distribution
in streams and rivers over every part of the land surfaces, could
not take place. Under these circumstances the unchecked evaporation
of the oceanic waters would probably go on, till the proportion
of water-vapour increased to such an extent in the atmosphere as
effectually to destroy those nicely-balanced conditions upon which the
continued existence of both vegetable and animal life depend.
But the repeated upward and downward movements which have been shown
to be going on in the great land-masses of the globe, giving rise
in turns to those lateral thrusts and tangential strains to which
mountain-chains owe their formation, afford a perfect compensation
to the action of the external forces ever operating upon the earth's
surface.
If, however, the uncompensated effect of the external forces acting on
the earth's crust is calculated to bring about the destruction of those
conditions upon which the existence of life depends, the uncompensated
effect of the internal forces acting on the earth's crust are fraught
with at least equal dangers to those necessary conditions.
[Sidenote: CONTRAST BETWEEN THE EARTH AND MOON.]
In our nearest neighbour among the planets--the moon--the telescope has
revealed to us the existence of a globe, in which the internal forces
have not been checked and controlled by the operation of any external
agencies--for the moon appears to be destitute of both atmosphere and
water.
Under these circumstances we find its surface, as we might expect, to
be composed of rocks which appear to be entirely of igneous origin;
the mountain-masses, unworn by rain or frost, river or glacier, being
of most prodigious dimensions as compared with those of our own globe,
while no features at all resembling valleys, or plains, or alluvial
flats are anywhere to be discerned upon the lunar surface.
But by the admirable balancing of the external and internal forces
on our own globe, the conditions necessary to animal and vegetable
existence are almost constantly maintained, and those interruptions
of such conditions, produced by hurricanes and floods, by volcanic
outbursts and earthquakes, may safely be regarded as the insignificant
accidents of what is, on the whole, a very perfectly working piece of
machinery.
The ancients loved to liken the earth to a living being--the macrocosm
of which man was the puny representative or microcosm; and when we
study the well-adapted interplay of the forces at work upon the
earth's crust, both from within and without, the analogy seems a
scarcely strained one. In the macrocosm and the microcosm alike, slight
interferences with the regular functions occasionally take place, and
both of them exhibit the traces of a past evolution and the germs of an
eventual decay.